Because it doesn't work very well. In particular, the C preprocessor doesn't know Perl syntax. Take this tiny example and save it into cpp.pl:

#define FOO "bar"
print qq(The value of FOO is "), FOO, qq("\n);
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$ perl cpp.pl
The value of FOO is "FOO"
$ perl -P cpp.pl
The value of "bar"  is "FOO"
$
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I expect that you could get many C preprocessors to choke or issue warnings on fairly common Perl syntax as well.

This is enough of a reason for me to not use such a trick. There are several other good reasons to not use this given in the thread. One of which I ran into just trying to demonstrate how my tiny example fails. First, on Win32 even with MS VC++ installed:

> perl -P cpp.pl
'cl' is not recognized as an internal or external command,
operable program or batch file.
> vcvars32
Setting environment for using Microsoft Visual C++ tools.
> perl -P cpp.pl
Command line error D2003 : missing source filename
>
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$ perl -P cpp.pl
ngcc: -: No such file or directory
ngcc: No input files specified.
$
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